The hypothesis upon which this project is based is that a major determinant of fibrillogenesis in AL amyloidosis is the nucleotide sequence of the rearranged immunoglobulin light chain gene in the clonal plasma cells of patients with this life-threatening disorder. This nucleotide sequence determines the primary amino acid sequence which we postulate is key for the pathologic properties of fibrillogenic proteins, including the folding of the protein itself, its post-translational modification, its association with accessory molecules including other proteins and proteoglycans, and its interactions with the tissues of the host. Other projects in this Program will focus on the latter properties; this project will focus on identifying the nucleotide sequence of the expressed mRNA and studying recombinant proteins in vitro and in vivo. We will clone and sequence light chain genes from patients' plasma cells, taking advantage of the unique patient population with this disease seeking consultation at Boston Medical Center. The rearranged amyloidogenic light chain gene sequences will be compared to germ line sequences, non-malignant light chain sequences, and to myeloma light chain sequences. In collaboration with colleagues at the Boston University Biomedical Engineering Research Center we will analyze these sequences, model the proteins encoded by them, and identify amino acid residues over-represented in fibril-forming light chains that are predicted to sere as structural determinants of fibrillogenesis. The contribution of these residues to fibrillogenesis will be assayed by expressing wild type and informative site-directed mutant light chains in vitro and examining their biochemical and biophysical properties in collaboration with Project 2 and the Cores. In addition to these in vitro assays, fibrillogenic light chains will be expressed in vivo using transfected cells and transgenic mice to determine the consequences of human amyloidogenic light chain production in the whole animal (n collaboration with Project 3 and Core A). The mice will sere as a whole animal model for testing hypotheses about AL amyloid pathogenesis; furthermore, in collaboration with Project 4, they will serve as a preclinical model for immunotherapy.